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Electricity Generation is a challenging environment with its emphasis on flexibility of operation needed to achieve frequent and rapid changes of load. In older stations, the operational environment often contains a variety of control systems of differing capability provided by different suppliers at different times. The economic and operability benefits of improvement can be substantial.

Geometric Process Control (GPC) with its emphasis on improving process understanding directly addresses the reduction of operating costs by reducing variability in process operations. It does this by allowing engineers, for the first time ever, to visualise and work with the Operating Envelopes of a process without requiring use of algebra, calculus, statistics, neural nets or any other advanced mathematical methods.

There is a unique envelope, involving possibly hundreds of variables, for any one set of operating objectives but an operating plant will have inadvertently explored many, many more in a series of ‘undesigned’ operating experiments during its lifetime. Being able to see the differences between these many envelopes across hundreds of process and result variables all at once allows choosing the envelope most appropriate for optimum operations. The chosen envelope immediately gives Operating Limits for all the process variables in the envelope. Operators can use these immediately with no change to their existing working practices. Operating inside these new consistent Operating Limits gives improved efficiency, higher yields, minimum recycle flows and contributes to longer equipment and catalyst life through smoother operation with less variability than previously.

The set of Operating Limits is actually an Operating Window because interaction effects between variables are largely ignored. It is much better than the Operating Windows some companies have in use today because the GPC method of finding all the limits at once instead of one at a time ensures consistency and the minimum enclosing hypercube around your operating objectives.

Operator High-Low alarms can then be set just outside the newly-found Operating Limits so that they help the operator to operate inside the Operating Window. You will find the new alarms to be much improved with lower annunciation rates, fewer false alarms and very, very few standing alarms on the operators alarm list display.

The removal of false alarms and the goodness of the new alarm limits in helping the operators to operate inside the Operating Window will allow the identification of further improvements in operating practices, automation and process control. Once made these improvements allow the Operating Envelope and Operating Objectives to be narrowed for further improvement in process performance. The on-going activity of monitoring, identifying opportunity and subsequently tightening envelopes to permanently capture the benefits is a major part of the Process Stewardship activity performed by Unit Process Engineers.

An Operating Window ignores interaction between variables and is consequently larger than the Operating Envelope it encloses. This means that there are regions inside the Window but outside the Envelope where the operating objectives will not be achieved and explains why actual Yields and Efficiencies are never as good as 100% of predicted Yields and Efficiencies. Practioners of single-variable Statistical Process Control (SPC) will already be aware of this as a limitation to the wider use of SPC. For many plants the yield from an Operating Window is acceptable and there is no desire to improve further. But for those where there is a desire for further improvement GPC provides the means to take variable interactions into account and create a much better model of the Operating Envelope as a multi-dimensional geometric solid. Operating inside the Envelope provides better yields and efficiencies and the operator sees a picture of the current position of the process operating point position inside the envelope with Alerts being raised on variables outside the operating envelope but inside the operating window and effectively delivering Predictive Alarming. Operators in both continuous and batch processes have found the display of their position within the envelope (and its surrounding window) to be easy and natural to use.

The Operating Envelope model also contributes to increased process availability by providing a very sensitive and highly multi-variable detector of events, usually in sufficient time for the operator to make small process changes to avoid the event. It has applicability in condition monitoring, in predictive maintenance and in early warning of the approach to frequently occurring ‘faults’ such as column flooding, the onset of compressor surge, paper breaks and sticking of drill bits when drilling oil and gas wells.
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